1. Field of the Invention
The present invention relates to a method for projecting a photoelectron image, more particularly, to a method for projecting a photoelectron image using a mask having a photoelectrically active surface.
2. Description of the Related Art
Photolithography is an important technique in the production of, for example, VLSI. Among the photolithography techniques, there are techniques of projecting fine patterning to wafers or the like. One of these is the "Direct-Stepping-on Wafer" (DSW) process, wherein a scaled down, e.g. 5:1 or 10:1, projection exposure process using light (having a wavelength of about 4000 .ANG.) is carried out using a reticle as an original print. However, the DSW process has a resolution limit of about 1 .mu.m.
In order to improve resolution, the electron beam exposure process and X-ray exposure process are well known.
In the electron beam exposure process, the electron beam is scanned with a point or a rectangular shape, thereby increasing the exposure time. The electron beam exposure process therefore has a small throughput.
The X-ray exposure process uses an X-ray having a wavelength of 1 to 10 .ANG. and a large-scale light source of, for example, 10 to 50 kW whereby proximity exposure is carried out. Thus, the X-ray exposure process requires a highly accurate aligner to support and align the mask and wafer. On this point, the X-ray exposure is similar to conventional optical exposure.
However, there are problems in that the X-ray exposure process requires high cost due to the large scale of the light source and considerations of mask materials in relationship with the absorption coefficient and wavelength of light source. Also, blurring occurs due to changes in the gap between the mask and wafer generated by wrapping of the wafers. One light source used for generating the X-ray is a sychrotron emittion light which is too large in scale and high in cost.
Recently, the ion beam exposure process has been studied. However, ion beam exposure has the same problems as electron beam exposure.
In an electron beam projecting process similar to the present invention, a mask image is projected to a portion to be patterned. The mask is produced by a process comprising the steps of providing a pattern to be projected made of an ultraviolet absorber, such as Cr, on, for example, crystal glass and covering the same with photoelectric material, such as CsI, from which electrons are emitted by irradiation with ultraviolet light. When ultraviolet light is irradiated from the back surface of the photoelectric material through the crystal glass (back illumination), photoelectrons are emitted only from the portion of the photoelectric material on the crystal glass, not from the portion of the photoelectric material on the ultraviolet absorber. The emitted electrons are focused on an electron sensitive resist provided on a wafer, while a magnetic field is applied therebetween.
However, the resist for the electron beam on the wafer is sometimes sensitized by the ultraviolet light, (particularly when it has a large energy) passed through the mask, as well as by the emitted electrons. This prevents accurate projections of the pattern of the mask, resulting in blurring of the image. Further, electrons of CsI are emitted as a result of ultraviolet light using a low pressure (5 mW) Hg lamp. Thus, the exposure period for 20 mm .quadrature. using the ultraviolet light is low, for example, 30 sec.